1. Field of the Invention
The present invention relates to a welding wire used for welding dual phase stainless steel, and more particularly, a flux cored wire which can form a welding zone excellent in weldability, crack resistance, stress-corrosion cracking resistance and pitting resistance in welding austenitic and ferritic stainless steel that is used in various apparatuses for the chemical industry such as a condenser pipe where the sea water is used as cooling water, a heat exchanger for the petroleum chemistry, an oil well pipe, etc.
2. Description of the Related Art
Stainless steel is a kind of alloyed steel where Cr is added into steel to enhance the corrosion resistance, and is mainly classified into Cr based steel and Crxe2x80x94Ni based steel according to composition and classified into five categories such as martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, austenitic-ferritic stainless steel and precipitation hardening stainless steel according to metal structure.
The martensitic and ferritic stainless steels are Cr based stainless steel, in which martensitic stainless steel with 13 Cr stainless steel as the representative one has corrosion resistance and heat resistance to be widely used as general machine structural steel, and ferritic stainless steel with 18 Cr stainless steel as the representative one is poor in mechanical properties since material quality cannot be enhanced via heat treatment. However the ferritic stainless steel is easy in welding due to having no hardenability, has ferromagnetic properties of the BCC structure, and is excellent in formability thus widely used in household articles or vehicle parts.
The austenitic stainless steel is a Crxe2x80x94Ni based stainless where 18-8 stainless steel is the representative one. The austenitic stainless steel is an industrially fundamental stainless steel which shows nonmagnetic properties of the FCC structure and is excellent in toughness, ductility and corrosion resistance, and from which variously enhanced steels are being developed.
The precipitation hardened stainless steel, where precipitation hardening elements are doped into the Crxe2x80x94Ni based stainless steel to enhance the strength while reducing the deformation due to the hardening heat treatment, is used for machine parts such as corrosion resistant spring, shaft, valve and impeller where good formability and high strength are necessary.
Also, the austenitic-ferrite based dual stainless steels contain 20 to 25 wt % Cr and 4 to 8 wt % Ni where Mo, Cu, N, etc. is added individually or as a mixture thereof to obtain the microstructure of a base matrix having 50 to 80 wt % ferrite and balancing amount of austenite and are excellent in sea water resistance and intergranular corrosion resistance to be widely used for various chemical industries where corrosion prevention against the sea water is necessary.
It has been relatively currently studied and developed about the dual phase stainless steel in order to enhance resistance against pitting and intergranular corrosion which are the disadvantages of the austenitic stainless steel while obtaining the advantages of the ferritic stainless steel of strong resistance against stress corrosion.
When the dual phase stainless steel contacts with an aqueous solution containing halogen ions such as Clxe2x88x92, Fxe2x88x92, etc., since there is a disadvantage that the halogen ions are absorbed to a passive state film(a tight adherent film of iron-chromium oxide) defect formed on the stainless steel surface so that the passive state film defect is destroyed to elute the metal and thus generate a pitting, that is why the welding zone properties are especially important.
However, there are problems that welding wires for austenitic stainless steel of the related art are poor in weldability to the dual phase stainless steel and have a difficulty in obtaining a welding zone corresponding to the properties of the dual phase stainless steel.
The present invention is proposed to solve several problems which are generated in welding a dual phase stainless steel by using a welding wire for stainless steel of the related art and therefore it is an object of the invention to provide a flux cored wire for welding the dual phase stainless steel in which a welding zone can be obtained that is high in strength and excellent in crack resistance, stress-corrosion cracking resistance and pitting resistance.